1. Field of the Invention
This invention relates to a cold shield and cold filter for infrared detector arrays.
2. Brief Description of the Prior Art
The sensitivity of infrared detectors depends upon both their signal response and upon the noise associated with the detection process. The most sensitive such detectors are photon detectors, that is, those detectors whose signal response is proportional to the number of photons incident thereon. The limit of performance of such detectors is set by the random fluctuations in the rate at which background photons are received. Detectors for which this is true are generally referred to as "background limited". The magnitude of the fluctuation energy is very nearly proportional to the total number of photons detected. It follows that, for photon detectors, the output signal-to-noise ratio is maximized by rejecting all radiation that does not contribute appreciably to the signal while accepting all radiation that does so contribute.
Radiation which contributes to the signal at the detector plane is defined in two ways: (1) geometrically and (2) spectrally. The geometric definition is determined by the optical system associated with the detector array. All energy gathering and imaging optical systems have an aperture and an aperture stop which determines how much energy is gathered by a detector element. The spectral definition is determined by the type of radiation source being detected, modified by the spectral transmission of the atmosphere intervening between the source and the optics and detector. Maximizing the signal requires that the vacuum chamber, generally a Dewar, be so constructed that as much of the geometric and spectral content of the signal as possible is accepted.
Maximizing the signal-to-noise ratio requires a balance between the amount of signal collected and the amount of noise generated by the total photon flux. Geometrically, both maximizing the signal collected and minimizing the non-signal radiation collected can be accomplished by making the aperture stop a physical part of the detector. Any surfaces that are within the view of the detector array outside the aperture stop must be cold and black (highly emissive and low reflecting) so that they produce a negligible photon flux at the detector array. These surfaces are usually cooled by the same cooling mechanism employed to cool the detector array. All known "background limited" photon detectors must be cooled to reduce other detector noise mechanisms to a level below that induced by the background flux. The photon detectors are mounted in thermal contact with the cooling means, are packaged within a vacuum space for thermal isolation and receive the signal radiation through an optical window placed in one wall of the vacuum space.
To eliminate radiation that contributes less to signal than to background because of the wavelength span it covers requires the use of a bandpass spectral filter. This filter must be placed so that no radiation having significant energy content at wavelengths outside the filter bandpass can reach the detector array.
The problem being addressed is that of configuring a detector array within a vacuum space, with an aperture or stop defining the geometrical limits and a spectral filter defining the spectral limits of the radiation the detector array can receive.
FIGS. 1a to 1d illustrate several prior art approaches to solving the problem as set forth above. The configuration of FIG. 1a includes a vacuum containing vessel having a vacuum wall 3, which can be a Dewar, having a detector array 4 therein. The side of the enclosure facing the detector array 4 includes a wall 7 of a material opaque to incoming radiation and which is secured to the vacuum wall 3. The wall 7 has an aperture 1 therein and a window 2 having a filter coating or coatings 5 thereon secured to the wall 7 and disposed over the aperture to provide the aperture with a filter thereover for incoming radiations to the detector array 4. This configuration places spectral filter coatings on one or both surfaces of the vacuum window 2 and places the aperture 1 in the interior surface of the window. The detector can receive radiation both emitted by the warm vacuum wall as well as reflected by the wall from other surfaces within the vacuum. This is a poor solution to the problem because a significant amount of radiation can reach the detector which originates external to the geometrical limits of the aperture 1 and external to the spectral limits of the filter coatings 5.
The configuration of FIG. 1b is the same as in FIG. 1a except that the filter coating 5 is disposed within the vacuum vessel and between the detector array 4 and the aperture 1. This configuration achieves a somewhat better result than that of FIG. 1a by placment of the filter coatings on a substrate immediately in front of the detector array. The coatings on the window can now become anti-reflectance coatings which provide high transmission over the filter spectral band. The spectral content of the radiation impinging on the detector array is now restricted to the desired pass band. The detector is still not restricted geometrically, however, so it can still receive spurious radiation by way of the vacuum wall. This configuration has the additional drawback that the filter substrate 8 adds mass to the cooled region, thereby adding to the time required to cool the detector array 4 to its operating temperature.
The configuration of FIG. 1c is an improved version of FIG. 1a except that the wall 7 is removed and the window 2 with filter coating 5 thereon is secured to the vacuum wall 3 directly to provide the vacuum vessel. Additionally, a baffle 6 is provided within the vacuum vessel and has an aperture 1 therein through which radiation travels to the detector array 4 at the surface of the baffle opposing the aperture. This configuration moves the aperture 1 to an enclosed shield or baffle 6 surrounding the detector array 4. Coating the interior surface of this baffle 6 with a highly emissive (therefore low reflecting) coating reduces the radiation reaching the detector array 4 from the baffle interior to a negligible amount. The aperture 1 of this baffle now becomes the aperture stop of the optical system, establishing a limit to the angles at which the detector array 4 can receive radiation. If the top, outer surface of the baffle 6 is highly emissive, this configuration achieves the desired geometric and spectral restriction of background radiation. If the surface is reflective, however, a significant amount of spurious radiation can reach the detector array 4 by multiple reflections between the window surfaces 2 and the top surface of the baffle. Since this radiation is not filtered by the window coatings, it will be at wavelengths outside the spectral bandpass of the window.
The major drawback of the configuration of FIG. 1c is the inaccessibility of the aperture 1. Since either the window substrate 2 or its coatings 5 are usually visibly opaque, the location of the aperture 1 cannot be determined by visible means. Another drawback is the necessity to strike a compromise between baffle rigidity and heat load imposed upon the cooling means. The baffle 6 must have sufficient mass to make it rigid enough to avoid motion of the aperture 1 with respect to the optical system, thereby modulating the background radiation reaching the detector array 4 and creating a spurious signal. Too much mass increases the time required to cool the baffle 6 and detector array 4 to operating temperature.
The configuration of FIG. 1d is a combination of the configurations of FIGS. 1b and 1c and uses the filter position of FIG. 1b and the aperture position of FIG. 1c. The drawbacks to this configuration are aperture inaccessibiltiy, lack of rigidity and excess cool-down time.